Plastic optical fibers (POF) are in practical use in lighting applications, sensors for FA, or media for transmission of information in the communications industry by taking advantages of POFs such as low cost, lightweight, flexibility, and large diameter. As core material of POFs, a polymethyl methacrylate (PMMA) resin having sufficient mechanical strength and transmission characteristics is used as a main component.
As for POF using PMMA as a core material, because PMMA has a glass transition temperature (Tg) of about 110° C., the POF can be used actually under temperatures of the upper limit of about 110° C. even when the outside of the POF is coated with a polymer having a higher heat resistance. Therefore, in applications that requires higher heat resistance, it has been attempted to use materials having higher heat resistance than PMMA as core material. Specifically, POFs have been proposed which contain any of the following various materials as core material: polycarbonate resins described in Patent Document 1 or Patent Document 2; or amorphous polyolefin resins having alicyclic groups with high heat resistance as main chains described in Patent Document 3, Patent Document 4, or Non-Patent Document 1.
However, when POFs with polycarbonate resins (PC resins) as core material are to be produced, it is difficult to purify the core materials and remove foreign matters and the like. These POFs further have large light scattering loss due to ununiformity of density of the polymers themselves. Therefore, POFs with PC resins as core material is much inferior in transmission characteristics (transmission loss: equal to or more than 500 dB/km) to POFs with PMMA as core material (transmission loss: 130 dB/km). In addition, homopolymers or copolymers of alkyl fluoride (meth)acrylates or polymers containing fluorinated ethylene, which are widely used as cladding materials for POFs, have low adhesion to polycarbonates. Therefore, POFs with polycarbonate as core material tend to cause structural changes such as separation at the core-clad interfaces.
In addition, polymers having alicyclic groups in the main chains commercially available from several companies for core materials of POFs are difficult to purify. POFs using the polymers as core material have suffered from problems such as inferior transmission characteristics, low adhesion between the core and the clad, or the like as well as the POFs with polycarbonate resins as core material.
Meanwhile, in Patent Document 5 and Patent Document 6, POFs having relatively good transmission characteristics are proposed in which copolymers of methacrylate having alicyclic groups as side chains such as bornyl methacrylate, adamantyl methacrylate or tricyclodecanyl methacrylate and methylmethacrylate (MMA) are used as core material.
However, as to POFs in which polymers having monomer units of (meth)acrylates having common alicyclic groups as side chains are used as core material, the alicyclic groups of the polymers tend to decompose and leave at/from sites of ester bonds when the polymers of the core materials pass through high temperature units maintained at 200° C. or higher such as extruders or nozzles. Therefore, such POFs have a problem that the polymers cause heat deterioration when the polymers are melted and formed, thereby deteriorating transmission characteristics. In addition, improvements of the POFs thus manufactured are demanded in terms of heat resistance.
Recently, in Patent Document 8 and Patent Document 9, POFs with homopolymers or copolymers as core material have been proposed where the homopolymers are produced from α-methylene-γ-butyrolactone derivatives, respectively, which provide the POFs with a good balance between transparency and heat resistance such as high glass transition temperature or resistance to thermal decomposition, such as α-methylene-γ-methyl-γ-butyrolactone, α-methylene-γ,γ-methyl-γ-butyrolactone, α-methylene-γ-ethyl-γ-butyrolactone, or α-methylene-β-methyl-γ-butyrolactone, and the copolymers are produced from these monomers and methacrylate monomers.
However, the homopolymers of α-methylene-γ-butyrolactone derivatives and the copolymers of the α-methylene-γ-butyrolactone derivatives and methacrylate monomers described in the documents do not have sufficient transparency for core materials of POFs. In addition, in order to make the copolymers to have glass transition temperatures as high as those of polycarbonates, it is necessary to copolymerize α-methylene-γ-butyrolactone derivative units to account for about 50% by mass of the copolymers. Therefore, the resulting copolymers have a problem of having a low mechanical strengths.
Furthermore, in Patent Document 7, it is reported that homopolymers of α-methylene-β-methyl-γ-butyrolactone or α-methylene-β-ethyl-γ-butyrolactone, copolymers derived from monomers of α-methylene-β-methyl-γ-butyrolactone, or α-methylene-β-ethyl-γ-butyrolactone and monomers of methacrylates are applicable to core materials for optical waveguides because the homopolymers and the copolymers have higher glass transition temperatures than PMMA, excellent transparency (light transmittance) and high indexes of refraction. However, performance of the homopolymers and the copolymers as POFs are not verified actually.
In general, in copolymerization system of an α-methylene-γ-butyrolactone derivative and a methacrylate monomer these monomers have a large difference of reactivity ratio which tends to produce block copolymers as described in Non-Patent Document 2 and Non-Patent Document 3. Furthermore, if there is a large difference in refraction index between the α-methylene-γ-butyrolactone derivative and the methacrylate monomer, such a copolymer will have a light scattering loss much larger than PMMA resins which are commonly used as core material of POFs. Therefore, it will not be easy to use the copolymer as core material of POFs or as transparent optical material as it is.
As for techniques for reducing light scattering loss of polymers, a technique of thermally melting PMMA has been reported as described in Non-Patent Document 4 and Non-Patent Document 5. However, this technique is applied to homopolymers and there is no known example in which use of this technique sufficiently reduces light scattering loss of the copolymers having a strong tendency of block copolymers or the copolymers having large difference of indexes of refraction between constituent monomers, and these copolymers are put to actual use as core material of POFs.    Patent Document 1: Japanese Patent Laid-Open No. 06-200004    Patent Document 2: Japanese Patent Laid-Open No. 06-200005    Patent Document 3: Japanese Patent Laid-Open No. 04-365003    Patent Document 4: Japanese Patent Laid-Open No. 2001-174647    Patent Document 5: Japanese Patent Laid-Open No. 63-74010    Patent Document 6: Japanese Patent Laid-Open No. 63-163306    Patent Document 7: Japanese Patent Laid-Open No. 08-231648    Patent Document 8: Japanese Patent Laid-Open No. 09-033735    Patent Document 9: Japanese Patent Laid-Open No. 09-033736    Non-Patent Document 1: Akira Tanaka, summary proceedings of 8th POF Consortium, POF Consortium, Apr. 26, 1995, p. 7 to 15    Non-Patent Document 2: Polymer, Vol. 21, 1215 to 1216 (1979)    Non-Patent Document 3: Journal of Polymer Science: Part A: Polymer chemistry, Vol. 41, 1759 to 1777 (2003)    Non-Patent Document 4: Japanese journal of polymer science and technology, Vol. 42, No. 4, 265 to 271 (1985)    Non-Patent Document 5: Japanese journal of polymer science and technology, Vol. 53, No. 10, 682 to 688 (1996)